The Electric Motorcycle: A DIY Primer: Page 4 of 5

Inside this Article

This 1984 Honda VF500F Interceptor donor bike has its frame stripped and primed, ready for paint.

This 1984 Honda VF500F Interceptor donor bike has its frame stripped and primed, ready for paint.

AC brushless motor.

PMDC brushed motor.

BLDC hub motor.

The controller, contactor (lower right), and fuse (upper right), with everything neatly laid out, will be hidden under the old fuel tank.

Both the charger and the controller generate some heat, but airflow around the aluminum mount will keep things cool. Note the two chargers: one is a 72 V charger for the main pack. The second, smaller one is a 12 V charger for this bike’s separate 12 V system battery.

Battery mounts are another place where hefty design and redundancy are wise.

An example of a basic motor mount with a Motenergy ME0709 motor. The mount bolts to the frame at the top and bottom, and a bar counters the rear pull of the chain.

Using duct tape to test component placement.

This mount has three sets of hold-down brackets, able to handle the 90 pounds of batteries.

Two 12 V SLA scooter batteries, wired in parallel to give enough capacity for lights and a horn.

The Cycle Analyst display tracks vehicle speed and the propulsion batteries, including voltage, power consumption, and remaining capacity.

The kill switch operates the main battery contactor to cut power in an emergency.

The throttle, which is electrically connected to the controller, is used to increase and decrease the power that goes to the motor.

The author’s son Tyler takes a spin on the newly converted electric motorcycle.

Advanced

Keeping It Under Control

You can use the standard controls that are on your donor bike, with the addition of a kill switch that controls the power to the contactor and a high-voltage main cutoff switch. Often, you’ll see the addition of some fairly sophisticated monitoring devices like the Cycle Analyst, which reports battery state of charge and estimated range, along with speed. It even serves as a data logger and provides GPS information for plotting data over the course of a ride. Some owners just run a simple voltmeter.

Lead-acid batteries can indicate state of charge by their voltage, to the practiced eye, but lithium-ion technologies need more sophisticated monitoring to provide an accurate state of charge. Because the voltage stays pretty high throughout the charge/discharge cycle, it’s not a very good indication of lithium SOC.

For this motorcycle, the throttle is a simple twist-grip Magura matched to the controller we’re using. It slips over the bars, just as the stock gas throttle did, and is controlling a fairly simple 0- to 5-ohm potentiometer inside the twist grip. This gives you direct electrical control of the motor speed controller, eliminating any cabling other than a relatively fine electrical wire.

The 12 V System

The battery propulsion pack voltage is too high to power the lights and horn, so you’ll need a 12 VDC power source. Do not just connect the lights to one of your 12 V batteries, since it will dramatically affect the balance of the pack, leading to premature battery failure.

A simple solution is to just add another battery, independent of the main pack. (Naturally, you’re going to have to add another simple 12 V charger for this, but a simple 12 V “wall-wart” type charger does nicely.) Another common and less heavy method is to add a DC/DC converter, which will take the high voltage of your main pack and step it down to 12 V.

The Controller/Contactor/Fuses

The controller, along with the contactor and the main fuse, is usually mounted on the top part of the frame and covered by the tank. Neatness translates into safety—if you give this some detailed consideration, there’s less chance of making the wrong connections and high-voltage connections are less likely to come undone. Well-routed wires also minimize chafing and breakage.

On a medium-sized bike, there’s likely room to mount the chargers onboard, in our case above the controller and contactor. This allows you to plug in wherever you park the bike to get an “opportunity charge.” Even if you give it just a little boost when you’re having your morning coffee and donut, it can make the difference between an exhilarating ride home or finding yourself babying the bike back to the garage.

Cabling

Typically, you need to use at least 4 AWG high-voltage cables for all of your propulsion applications. In some cases, 2 AWG is used, and if you have doubt, err on the side of bigger. If your cables heat up at all in use, they certainly need to be increased.

It’s essential to work with a good wiring diagram. You may be able to find information on various wiring configurations on common EV controller manufacturer websites. The Alltrax AXE controller site, by far one of the most commonly used, is where we started, and then made slight modifications.

Getting Help

The devil is in the details, and as you build your electric motorcycle, you’re undoubtedly going to have questions and challenges. The best place to look for help (and do your basic research) is in online communities. ElMoto.net is a very authoritative online group for electric motorcycles—a great group of generous people with vast experience. For general EV technology, Endless Sphere Technology has an enormous amount of information.

Sign up, and start reading. Almost every question will probably already have been asked and answered, so start with the online search feature. Once you get a feel for the group, introduce yourself and your project, and ask away. Just prepare to be bombarded with advice and opinions. Be prepared with photos of your build, too. As the saying is often repeated there, “Photos, or it didn’t happen!”

Another great resource is the EV Album site where you can browse hundreds of projects like your own, and review the specifications, component combinations, and performance results. There’s a lot to be learned from seeing what others have done—what’s worked, and what hasn’t.

Comments (2)

The one thing missed and the biggest factor in EV MC range is aerodynamics. A standard MC has the aero of a brick!! And that at over 25mph just sucks power.

But fairly simple aero mods can double range. Also lowering the seat as much as you can helps lower frontal area.

Even a box behind the rider with curved front corners wider than the rider and gently curving back inward before being chopped off cleanly plus going down to the axle level can seriously cut drag and give lockable space for shopping, etc, by cleaning up the airflow, thus cutting drag.

I'm building a complete aero cabin on my MC to make it a long distance cruiser at 70 mph. Just got the chassis running on it's own power as should be finished by mid Feb with the aero cabin.

Awesome points you make about aerodynamics. Once, back in my bicycle racing days I heard that, at 30 miles per hour, a cyclist is using 90% of their energy just pushing the wind, and only 10% moving themselves and the bike. (or, if there was a 30 mph headwind, they'd be using 90% of their energy just to stay upright). I'm guessing that an MC will have a similar aerodynamic profile...if not worse.

Another thing to remember is that (like the wind turbine guys say), the power in the wind is a cube of its velocity. I assume that it's the same ratio working the other direction...pushing through the wind. So as your speed goes up, the energy that it takes to push through the air is going up at a cube of that. IE going from 20 mph to 30 mph is a 150% increase in speed, but a 337% increase in required power (1.5 x 1.5 x 1.5 = 3.375). So, reducing speed is the best way to reduce required energy. Albeit, who want to slow down on a motorcycle.